There is a great need in the semiconductor industry for metrology equipment that can provide high resolution, nondestructive evaluation of product wafers as they pass through various fabrication stages. In recent years, a number of products have been developed for the nondestructive evaluation of semiconductor samples. One such product has been successfully marketed by the assignee herein under the trademark Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos.: 4,634,290; 4,646,088; 5,854,710; 5,074,669 and 5,978,074. Each of these patents is incorporated in this document by reference.
In the basic device described in the patents, an intensity modulated pump laser beam is focused on the surface of a sample for periodically exciting the sample. In the case of a semiconductor, thermal and plasma waves are generated in the sample that spread out from the pump beam spot. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
In the basic device, a second laser is provided for generating a probe beam of radiation. This probe beam is focused colinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of reflected probe beam. The photodetector generates an output signal that is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation frequency. In the preferred embodiment, a lock-in detector is used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity (MOR) of the sample.
In the early commercial embodiments of the Therma-Probe device, the pump and probe laser beams were generated by gas discharge lasers. Specifically, an argon-ion laser emitting a wavelength of 488 nm was used as the pump source. A helium neon laser operating at 633 nm was used as the probe source. More recently, solid-state laser diodes have been used and are generally more reliable and have a longer lifetime than the gas discharge lasers. In the current commercial embodiment, the pump laser operates at 780 nm while the probe laser operates at 670 nm.
In practice, the response of the sample to the pump beam is dependent to some degree on the wavelength. Further, the sensitivity of the system is also dependent on either pump or probe beam wavelength and the relationship between the pump and probe beam wavelengths. The combination of wavelengths selected by the assignee in its commercial embodiment is intended to strike a balance allowing measurements over a relatively broad range of samples. However, it can be shown that certain samples could be more accurately measured if the pump and probe beam wavelengths were optimized for that sample type or sample range.
In the most common commercial application of the Therma-Probe, the density or dosage levels of implants in silicon are measured. While the current pump and probe beam wavelengths provide good sensitivity across a relatively wide range of doses, certain regions are less sensitive than others. Accordingly, it would be a benefit if the user was permitted to select a particular set of wavelengths to perform certain measurements.